It is known that a Hall effect sensor passing an energizing current (or equivalently subjected to a voltage) and subjected to a magnetic field perpendicular to the direction of current flow generates a so-called Hall voltage at its terminals in a third direction in space perpendicular to the directions of the magnetic field and the current flow.
The Hall voltage V.sub.H is proportional to the product of the strength I.sub.a of the energizing current (or equivalently the amplitude of the energizing voltage V.sub.a) and the amplitude of the magnetic field B. In an ideal sensor we thus have: EQU V.sub.H =K.sub.i .multidot.B.multidot.I.sub.a =K.sub.v .multidot.B.multidot.V.sub.a ( 1)
with K.sub.i =1/(e.multidot.n.sub.s), where e is the charge on an electron and n.sub.s is the density per unit area of the charge carriers and K.sub.v =(W/L).mu., where W and L are the width and the length of the sensor and .mu. is the mobility of the charge carriers. In a real Hall effect sensor having inhomogeneities or misalignment of the electrodes inherent in the manufacturing process, the Hall voltage is written: EQU V.sub.H =K.sub.i .multidot.I.sub.a .multidot.B(1+f/.mu.B) (2)
where the parameter f, small compared with 1, takes account of the departure from the ideal situation.
In order to obtain good sensitivity, a low carrier density per unit area is necessary.
This may be effected for example using a semiconductor structure comprising a homogeneous conductive layer of small thickness (in the order of a micrometer). This layer may be a layer of silicon for example, doped n-type on a substrate of p-type, or equally a layer of GaAs doped n-type on a semi-insulating GaAs substrate.
It may equally be effected using a hetero-structure semiconductor comprising at least two superimposed layers of semiconductor material having forbidden bands of different widths and selectively doped in the material with the widest forbidden band.
In this case, the carriers are confined to a very narrow region (whose thickness may be from 10 .ANG. to 100 .ANG.) located at the interface between the two materials, on the side of the material with the smallest forbidden band.
The sensor of the invention is of this second type.
The reference U.S. Pat. No. 4,912,451 describes a heterostructure comprising at least two quantum wells: a first quantum well formed at the interface between two semiconductor layers with forbidden bands of different widths and in which there is a two-dimensional electron gas, and a second quantum well whose "sub-band" has an energy level in its fundamental state higher than that of the two-dimensional electron gas.
This hetero-structure has a high sensitivity as a Hall effect sensor. It does not saturate, even under the influence of a strong electric field and it provides a strong output signal even in this case. However, this hetero-structure is sensitive to temperature variations, which affect the output signal. This Hall effect sensor has to be connected to a sophisticated and expensive electronic unit to compensate for the drift in the output signal when the temperature varies.
The article "AlGaAs:GaAs heterojunction Hall device" by T. Taguchi, published in Electronics and Communications in Japan, Part 2, Vol. 71, No. 3, 1988, pp 110-115, describes a simple AlGaAs/GaAs heterojunction used as a magnetic field sensor. As above, this structure may have increased sensitivity but it does not have good thermal characteristics. If made to operate at ambient temperature, it requires an electronic compensation unit if the temperature is subject to variations. By way of example, the coefficient of sensitivity to temperature mentioned in that article is 6800 ppm/.degree.C.
Another article "Highly sensitive 2DEG Hall device made of pseudomorphic In.sub.0.52 Al.sub.0.48 As/In.sub.0.8 Ga.sub.0.2 As heterostructure" authored by Sugiyanama et al. and published in "International Conference on Solid-State Sensors and Actuators", Transducers '91, San Francisco, Digest of Technical Papers, IEEE Publication Office, describes a pseudomorphic In.sub.0.52 Al.sub.0.48 As/In.sub.0.8 Ga.sub.0.2 As heterostructure deposited on an InP substrate. Although the performance as a function of temperature is improved compared with that of the two preceding references, it remains inadequate for application to an electricity meter, for which the required specifications are very demanding. By way of example, the coefficient of sensitivity to temperature of such a structure is 350 ppm/.degree.C. over a temperature range not exceeding 55.degree. C.